Set your oven to 350°F (175°C). Grease two 9-inches round cake pans and dust them with flour, or line them with parchment paper.

In a large mixing bowl, beat softened butter and sugar until light and fluffy. Add eggs one by one, beating well after each addition. Stir in vanilla, lemon juice, and lemon zest. In another bowl, sift together flour, baking powder, and salt.

Add the dry ingredients and milk to the butter mixture alternately, beginning and ending with the dry ingredients. Stir gently until just combined.

Divide the batter evenly between the prepared pans. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn out onto a wire rack to cool completely.

Whisk sugar, cornstarch, and egg yolks in a saucepan until smooth. Slowly add water, lemon juice, and zest. Cook over medium heat, stirring constantly, until thick and boiling. Boil for 1 minute, then remove from heat. Stir in butter until smooth. Allow to cool.

In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar while beating until stiff peaks form. Mix in vanilla.

Place one cake layer on a serving plate. Spread lemon curd evenly over it. Place the second layer on top. Spread meringue over the top and sides, shaping peaks or swirls with a spatula.

Return the cake to the oven and bake for 8-10 minutes at 350°F (175°C), until the meringue turns golden. Watch closely to avoid burning.

Let the cake cool completely before slicing to let the lemon curd and meringue set properly.